You are asked to prepare a buffer of pH 4.50 for control of a chemical reaction.

Question

You are asked to prepare a buffer of pH 4.50 for control of a chemical reaction. Using the table of pKas in the experimental procedure (Part 4), select an appropriate acid/base pair and clearly state the pair that you selected. Calculate the volume of the acid solution and the volume of the conjugate base solution that would be needed to prepare a buffer with a total volume of 200. mL. Assume all stock solutions are 1.00 M. Show all of your work.

From the table of pKas, I was able to figure out that acetic acid (CH3COOH) and acetate (CH3COO-) are the acid/base pair because the pKa is 4.75, the closest one in the table to the pH of 4.5. And now I just don't know how to go about finding the volume.

Explanation / Answer

Acetic acid and Sodium acetate buffer is the suitable for obtain pH = 4.5

First we have to decide what should be the ratio of molaity of acetic acid and sodium acetate. For the we have to use Henderson - Hasselbalch equation

pH = pKa + log ([ A-]/[HA])

Where, [A-] = Concentration of conjucate base ( acetate)

[ HA] =concentration of acid (acetic acid)

4.5 = 4.75 + log([A-]/[HA])

log ([A-]/[HA]) = -0.25

[A-]/[HA] = 0.562

If we need to prepare 1M buffer solution

[A-] + [ HA] = 1

[A-] = 0.562[ HA]

0.562[HA] + [HA] = 1

1.562[HA] = 1

[HA] = 1/1.562 = 0.640M

Therefore, [A-] = 1-0.640 = 0.360M

Our stock solution of Acetic acid is 1M, from this we have to prepare 0.640 M for 200ml . Applying the dilution formula.

C1 × V1 = C2 × V2

1M × V1 = 0.640M × 200ml

V1 = 128ml

So,we need to take128 ml from 1M Acetic acid stock solution

Similarly,

Sodium acetate stock solution is 1M, from this we need prepare 0.360M for 200ml . Applying the dilution formula

C1 × V1 = C2 × V2

1M × V1 = 0.360M × 200ml

V1 = 72ml

So, we need to take 78ml from 1M Sodium acetate stock solution

Therefore, 128ml of Acetic acid solution of 1M and 72ml of Sodium acetate solution of 1M should be mixed to prepare 4.5 pH acetic acid and sodium acetate buffer solution for 200ml.

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